Method for reducing the mercaptan sulfur content of hydrocarbon oils



Nov. 2, 1948.

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METHOD FOR REDUCING THE MERCAP'IAN SULFUR CONTENT OF HYDROCARBON OILS . John mpel, Brooklyn, and Stephen P. Cauley,

Jackson Heights, N. Y., assign ,Vacutun Oil Company,

tion of New York or: to Socony- Incorporated, a corpora- Application June 11, 194:, Sam No. 490,492

(oi. its-sc l This application is a continuationin-part oi our co-pending application Serial No. 463,602, filed October28, 1942, Treatment of hydrocarbon oils.

This invention is directed to the hydrocarbon oils for the reduction of the-mere captan sulfur content thereof, particularly'in now abandoned, for thetreatment of mercaptansfremaining inthegasoli'ne after the tended to provide a finished gasoline fraction; which is either sweet to the doctor" test or is sufficiently low in suliur to be acceptable under. specifications wherein the doctor test isnot used and total sulfur content is determined.

In treating hydrocarbons, such as straight gasoline, natural gasoline, cracked gasoline and the like, to render them'sweet to-the doctor test, the objective is to remove materials of the nature of hydrogen sulfide, mercaptans or the like or to convert them' to. innocuous compounds.

- Washing with aqueous solutions of strong alkalies', such as caustic'soda will removemost of the hy'- drogen sulfide and some, but not-all, of the mercapta'ns. somematerial hasbeen added as a soiutizer, or

solubility promoter, enables the caustic to dissolve and remove more or all of the mercaptans. Butyiene glycols, butyric acids, and phenolic'material are so used. Older methods make use'of sodium plumbite formed by dissolving litharge in caustic soda, the effect here being to convert mercaptans to mercaptides andlater to disulfldes, which are innocuous to the doctor test, by addition of free sulfur. v

In general, all processes of this kind leave something to be desired,- and this field, one of the oldest in petroleum refining, is still the subject of active research.

This invention has for its object the provision of a method for treatment of light petroleum hydrocarbons of the nature of gasoline, natural gasoline, cracked gasoline, kerosenes, solvents and the like, capable of efiecting very considerable reductions in the mercaptan sulfur content of the material so treated.

This invention is based upon the discovery that if small percentages oi' tannic acid be added to caustic solutions containing other-quite competent solutizers, a very considerable increase in the mercaptan dissolving power of the reagent occurs.

These desiredobjects may be effected by such a process when conducted as explained in the following discussion, certain features of which are shown in the drawing attached hereto, the single figure of which drawing presents in graphic form certain features of the process. In preparation for the discussion of the process, it may be pointed Washing with aqueo'us caustic to which 10% caustic solution. It is found to be quite high with low molecular weight mercaptans but decreases rapidly to rather impractical limits with mercaptans containing more than six carbon I cient- Kq which reagent and gasoline have t'actto a sufllcient degree and iora suflicient time a out that the criterion of removal of material from 1 g the oil phase to the reactant phase in a liquidliquid treating operation is the distribution coefllcaptans in the :reagent; by the concentration "of been brought in con- .to arrive at equilibrium.

left and the number of-carbon atoms in the mer is, the'efllciency in'removal of mercaptans ota atoms, ,aptly illustrating the reason why treat- ,ment withstraight caustic solutions has never 1 ,been particularly effective upon certain types of gasoline; V In. curve B the same 10% caustic solution is used except that to. the caustic there has been added 1% of tannic acid, As shown, this reagent is nearly'three times as effective for the removal .of C mercaptan as is the straight caustic solution. and this reagent retains a fairly good eflectivefiess' even for much heavier mercaptans. Curve C illustrates the effect of a solutiizer material such as, for example, o-cresol, the reagent used in curve 0 being a 5.0 normal sodium hydroxide solution which also contains sufficient additional sodium and o-cresylic acid to provide a strength of 2 normal sodium o-cresylate. As

may be observed, this reagent shown in-curv-e C effective as a straight 10% caustic for'the removal of a C5 mercaptan.

Curve D shows the effect of an addition of 1% of tannic acid to this samereagent as used in curve C. The removal efficiency is considerably increased. For example, for a C5 mercaptan the new solution is more than 50%more effective.

These results are contrary to those which might be-expected from study of the capability of tannic acid in caustic alone. In caustic alone in the absence of other solutizers, tannic acid does .not at all increase the capability for the removal coeificient' in the present caseis. arrived at by=dividingthe concentration of (mar- Turning toFigure'l of the sedrawings, fweflnd curves 4, B, C', and D, in which the distribution 'coeillcient isshown 'upon' the vertical scale at the where tannic acid was capable of effecting almost 1 I a tripling of the removal eiilciency of plain caustic, it increased the solutizer solution efflciency about 56% and with heavier mercaptans, where its increase in the efliciency of plain caustic is most pronounced, the percentage of increase with solutizers is less marked. Thus we have the rather surprising fact that while tannic acid ap-' parently does not enhance the activity of straight caustic for removal of lower molecular weight mercaptans, it is in this same area whereit ei iects its most marked increase in the eiiiciency of solutizer-caustic solutions.

Consequently, the present invention advances the fact that the presence of a small amount of tannic acid will very considerably increase the extraction power of an alkali metal hydroxide solution in the presence of added solubility promoters such as cresylic acid, isobutyric acid and the like, enabling avery material reduction in the amount of sulfur in the gasoline since the treating reagent after treatment may be physically separated from the gasoline. The caustic reagent used and the added solutizer therein may be any of those already widely known. In general, the caustic solution which is used may be one of relatively high concentration, such as solutions containing from 10-45% of caustic soda together with suiiicient of the solubility promoter or solutizer to achieve a desired efiectiveness in mercaptan removal. For example, a preferred reagent utilizing isobutyric acid will be a 40 B.

caustic solution to which has been added 20% by i weight of isobutyric acid. In the ease of a cresol.

containing reagent, a 5 normal caustic, 2 normal sodium cresylate reagent appears to be highly useful.

The caustic solution may be either sodium hydroxide or potassium hydroxide and it will usually be a'solution of a concentration between about caustic and 45% caustic. From the standpoint of efficiency, potassium hydroxide is to be preferred, since mercaptan solubility promoters are more effective in solutions of this-alkali. However, cost relationships will probably more usually indicate sodium hydroxide as the alkali to be used. The mercaptan solubility promoters to be used are any of the well known materials developed for this use and in the form in which they appear in the reagent are usually the alkali compounds of the .named promoters. Among solubility promoters which may be used are: ethylene glycol, ethanolamine, propanol amine, phenyl acetic acid, alpha hydroxy Valerie acid, butyric acid, isobutyric acid, propionic acid, isobutylene glycol and alkyl phenolic acids. The amount of-these solubility promoters which can be used depends only upon the solubility of the promoter itself in the caustic alkali in which it is to be used. In general, the beneficial efiect of the solubility promoter is increasedby an increase in concentration of the mercaptan solubility promoter, or of the alkali or of both. These mercaptan solubility promoters constitute a class of reagents which by this time are quite well known for the purpose. As will be observed from the list they fall into several kinds of chemical compounds. However, it is believed that materials of this kind are sufficiently well known to the art that they may be generically defined herein and in the following claims as solubility promoters capable of increasing the solubility of mercaptans in alkali metal hydroxide reagents.

The tannic acid may be C. P.'tannic acid, commercial tannic acid, tannin or any of the usual forms in which tannin appears .upon the market.

' The term tannic acid or tannin is understood Most of them are substances widely distributed to cover those substances generally so grouped.

in the vegetable kingdom. They are soluble in water, possess an acid, astringent taste, are colanimalfihides. The'constitution 0! many tannic acids is stillsomewhat obscure. Some appear to ored dark blue'or green byferrous salts, precipitategelatin, andfo'rm leather when applied to be glucosides of gallic acid, and decompose into gallic acid and sugar upon boiling with dilute ,acids. Otherscontain phloroglucin. Upon fusing with KOH these mostly form protocatechuic acid and phloroglucin. Among the tannic acid compounds there may be named gallo-tannic acid, digallic acid, gallyl-gallic acid, and the various tannin materials'derived from oak-bark, quininethe oil to be treated in any conventional treating bark, coffee, moringa-tannin, cutch, chestnut, logwood, sumac, and many other natural sources. Many such compositions appear to'contain compounds of the nature of polyhydroxyflavpinacols,

yielding, upon fusion with alkali, polyhydroxy benzenes such as phloroglucin, resorcinol, or pyrogallol, and polyhydroxy benzene carboxylic acids such as protocatechuic or gallic acid. All such compounds are considered to be equivalent and embraced by the term tannic acid as herein used.

The concentration of tannin in the treating solution may vary. It has been found that from 1% to 3% by weight appears to be the most efiective concentration, although concentrations-"as low as'0.1% maybe used. On the upper end,

concentrations higher than 5.0% appear at ent to yield diminishing retumsl The treatin solution may be contacted with pressystem, such as in a packed toweror any system providing countercurrently arranged contacting stages. The amount of solution to. be used may be varied to the individual treatingrequirements,

but in general, will be from 1-50% by volume of the oil being treated. The treating temperature may vary. The removal effect appearstobe at a minimum at 70 F. and increasesat temperatures above and below this temperature, at least withinthe limits of 32 F. and F. For practical operation, a] temperature of 75-100 F. is

recommended. Since this is quite a usual atmospheric temperature or readily attainable around an oil refinery, this phase of the operation presents no difiiculties.

We claim:

1. That method of treating a light petroleum fraction to reduce the mercaptan sulfur content thereof comprising contacting the petroleum fraction with an aqueous alkali metal hydroxide to which there has been added a solubility promoter other than tannic acid capable of increasing the ability of the hydroxide to dissolve mercaptans, and to which there has also been added a small amount of tannic acid suflicient to. further increase the mercaptan removal ability of the hydroxide more than additive and separating the oil from the mercaptan containing reagent 2. That method of treating a light petroleum fraction to reduce the mercaptan'sulfur content thereof comprising contacting the petroleum fraction with an aqueous alkali metal hydroxide to which there has been added a solubility promoter other than tannic acid capable of increasing the ability oi the hydroxide to dissolve mercaptans, and to which there has also been added a small amount, not in excess of about 5% by weight, of tannic acid suflicient to further increase the mercaptan removal ability of the hydroxide more than additive and separating the oil from the mercaptan containing reagent.

3. That method of treating a light petroleum fraction to reduce the mercaptan sulfur content thereof comprising contacting the petroleum fraction with an aqueous alkali metal hydroxide to which there has been added a solubility promoter other than tannic acid capable of increasing the ability of the hydroxideto dissolve mercaptans, and to which therefhas also been added a small amount, from about 1% to about 3% by weight, of tannic acid sufilcient to further increase the mercaptan removal ability of the hydroxide more than additive and separating the oil from the mercaptan containing reagent.

4. That method of treating light petroleum fractions to reduce the mercaptan-sulfur content thereof which comprises contacting a petroleum fraction with an aqueous solution of alkali metal hydroxide containing alkali metal 'cresylate and tannic acid, the distribution coefficient of said solution being greater than the sum of the distribution coeiiicient of aqueous amount of cresylate and the distribution coeflicient of aqueous alkali metal hydroxide containing the same amount of tannic acid,-and separating the petroleum traction from the mercaptan-containing reagent.

5. That method 01' treating light petroleum fractions to reduce the mercaptan-suliur content thereof which comprises contacting a petroleum fraction with an aqueous solution of alkali metal hydroxide containing potassium isobutyrate and tannic acid, the distribution coemcient 01 alkali metal hydroxide containing the same go REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 28 Number Name Date 2,228,028 Brower Jan. '1, 1941 2,228,041 YabroiI et a]. 1 Jan. 7, 1941 2,297,621 Henderson et al. Sept. 29, 1942 2,315,530 Loyd Apr. 6, 1943 2,316,092 Loyd Apr. 6, 1943 2,316,966 McNamara et al. Apr. 20, 1943 2,369,771 Bond Feb. 20, 1945 

